In the manufacture of patterned devices such as semiconductor chips and chip carriers, the steps of etching different layers which constitute the finished product are among the most critical and crucial steps involved.
In semiconductor manufacturing, optical lithography has been the main stream approach to pattern semiconductor devices. In typical prior art lithography processes, UV light is projected onto a silicon wafer coated with a layer of photosensitive resist through a mask that defines a particular circuitry pattern. Exposure to UV light, followed by subsequent baking, induces a photochemical reaction, which changes the solubility of the exposed regions of the photosensitive resist. Thereafter, an appropriate developer, typically an aqueous base solution, is used to selectively remove the resist either in the exposed regions (positive-tone resists) or in the unexposed region (negative-tone resists). The pattern thus defined is then imprinted on the silicon wafer by etching away the regions that are not protected by the resist with a dry or wet etch process.
Resists are generally comprised of a polymeric matrix, a radiation sensitive component, a casting solvent, and other performance enhancing additives. The polymeric portion of the resist should have a reasonable absorption at exposure wavelengths, and the resist composition must also possess suitable chemical and mechanical properties to enable transfer of the image from the patterned resist to an underlying substrate layer(s). Thus, an important parameter to be considered in the design of a resist material is the dissolution behavior of the material in the given developer. A patternwise exposed negative resist must be capable of appropriate dissolution response (i.e., selective dissolution in developer of unexposed areas) to yield the desired resist structure. The industry has largely supported the use of 0.263 N tetramethyl ammonium hydroxide (TMAH) as a developer for resist.
For a negative photoresist to work, the resist should have a reasonable dissolution rate before exposure and little or no dissolution after exposure. To achieve the required dissolution rate in aqueous base developer, acidic groups have been incorporated into the polymer structure. For example, hydroxystyrene has been widely used as such an acidic group in negative resist designed for use with irradiation at a wavelength of 248 nm. Hydroxystyrene, however, is too absorbing at 193 nm wavelength. Other acidic groups such as carboxylic acid (—COOH) and hexafluoroalcohol (HFA) have been used in 193 nm negative resist design. Carboxylic acid, however, is too acidic. Resist polymers with —COOH tend to dissolve very fast in 0.263 N TMAH developer before exposure and swell after exposure, which largely deteriorates the lithographic performance of the resist. The HFA group has a much weaker acidity than —COOH, and resists based on HFA have better dissolution properties. On the other hand, due to the high fluorine content in the HFA group, the etch resistance is often a concern.
Thus, there remains a need in the art for a negative resist composition that exhibits excellent dissolution response in aqueous base developer, and yet overcomes the above-mentioned problems associated with prior art resist compositions.